1. Field of the Invention
The present invention relates to a magnetoresistive element and a method of manufacturing the same, and to a thin-film magnetic head, a head gimbal assembly, a head arm assembly and a magnetic disk drive each of which incorporates the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect, and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. Typically, a spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a ferromagnetic layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter called a sense current) is fed in the direction parallel to the plane of each layer making up the GMR element. Such a structure is called a current-in-plane (CIP) structure. On the other hand, developments have been made for another type of GMR heads each having a structure in which the sense current is fed in a direction intersecting the plane of each layer making up the GMR element, such as the direction perpendicular to the plane of each layer making up the GMR element. Such a structure is called a current-perpendicular-to-plane (CPP) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element.
For a conventional CPP-GMR element, a CoFe alloy and an NiFe alloy have been mostly used as the material of the pinned layer and the free layer. In such a conventional CPP-GMR element, with regard to the configuration of layers capable of achieving a practical read gap length, the magnetoresistance change ratio (hereinafter called an MR ratio), which is a ratio of magnetoresistance change with respect to the resistance, is not more than approximately four percent and is therefore is insufficient in practice.
It is assumed that the low MR ratio of the above-mentioned conventional CPP-GMR element is attributable to a low spin polarization of the CoFe alloy or the NiFe alloy used as the material of the pinned layer and the free layer.
To increase the MR ratio, it has been proposed recently to employ CPP-GMR elements in which a half metal whose spin polarization is higher than a conventional metal such as a CoFe alloy is used as the material of the pinned layer and/or the free layer. JP 10-177705A discloses a GMR element in which a Heusler alloy, which is a type of half metal, is used as the material of the pinned layer and/or the free layer. Furthermore, JP 2005-116703A discloses a CPP-GMR element in which at least one of the pinned layer and the free layer includes a Heusler alloy layer.
The Heusler alloy will now be briefly described. The Heusler alloy is a term generally used for ordered alloys having a chemical composition of XYZ or X2YZ. An ordered alloy having a chemical composition of XYZ is called a half Heusler alloy. An ordered alloy having a chemical composition of X2YZ is called a full Heusler alloy. Here, X is an element selected from the group consisting of the transition metals of the Fe family, the Co family, the Ni family and the Cu family of the periodic table, and the noble metals. Y is at least one element selected from the group consisting of Fe and the transition metals of the Ti family, the V family, the Cr family and the Mn family of the periodic table. Z is at least one element selected from the group consisting of the typical elements of the periods from the third to fifth periods inclusive of the periodic table.
There is a possibility that the MR ratio of a CPP-GMR element may be greatly increased by using a Heusler alloy layer as the pinned layer and/or the free layer. Conventionally, however, even if a CPP-GMR element in which a Heusler alloy layer is used as the pinned layer and/or the free layer is actually fabricated, the MR ratio thus obtained is not more than approximately 5 percent. It is assumed that one of the reasons relates to heat treatment performed when the Heusler alloy layer is formed. This will now be described in detail. Typically, a Heusler alloy layer is formed by making a film to be the Heusler alloy layer and then performing heat treatment on this film to change the crystal structure of the film into one that achieves a high spin polarization. However, in a CPP-GMR element in which a Heusler alloy layer is used as the pinned layer and/or the free layer, the material forming the nonmagnetic conductive layer diffuses into the Heusler alloy layer in the course of the above-mentioned heat treatment. It is assumed that, through this diffusion, the roughness of a surface of the Heusler alloy layer closer to the nonmagnetic conductive layer is thereby increased and/or the regularity of the crystal structure is degraded in a portion near the surface of the Heusler alloy layer closer to the nonmagnetic conductive layer. As a result, it is assumed that the spin polarization is reduced in the portion near the surface of the Heusler alloy layer closer to the nonmagnetic conductive layer, and that the MR ratio is thereby reduced.